A piston normally has a cylindrical outer surface that is perfectly complementary and coaxial with the cylindrical inner surface of a cylinder in which the piston moves axially, with or without rotation of the piston about the axis. Obviously it is essential that no fluid flow be possible around the piston, that is axially between the piston outer surface and cylinder inner surface. Furthermore it is always essential that the piston remain perfectly square and centered within the cylinder.
To this end the piston is normally provided with a seal that prevents leakage, and with a scraper that prevents particles or the like that have somehow come to adhere to the cylinder wall from coming into contact with the fragile and accurately dimensioned seal. Furthermore either the piston itself is very accurately dimensioned to guide itself in the cylinder, or special guides are provided on the piston to keep it perfectly centered.
A double-acting piston must, of course, be provided with two seals and scrapers. The seals are normally constituted as soft elastomeric glands of U-section with one leg bearing inwardly on the base of the respective piston groove and another leg bearing outwardly on the inner cylinder surface. Such multiplication of parts even further complicates the structure.
The piston must therefore normally be built with a plurality of grooves or steps formed to hold the guides, seals, and scrapers. When no separate guide is provided the piston must be accurately machined to fit precisely in the cylinder bore. Obviously such construction is expensive. Furthermore the labor entailed in servicing any of these wear-prone parts is usually so excessive that the mechanic normally also replaces all of the others while the piston is out.